RJ100 and David, many thanks for these. Indeed DHL were happy with a post code but not a PO box, and I suspect Waterside is so big that it has its own couriers office.
I note that the Advocates web page has mostly contacts for the USA, indeed that is what you find with a plain web search. Perhaps America requires either legally or culturally to be able to get to a company, to serve papers or simply to speak to a customer service agent.